Amplifier



Patented Aug. 27, 1929.

UNITED STATES JOSEPH BETHENOD,

F PARIS, FRANCE.

AMPLIFIER.

Application led October 25, 1923, Serial No.- 670;798, and in France November 14, 1922.

For the purpose of reinforcing the telephonic current generated by means of a microphone the suggestion has previously been made to use amplifiers of various types especially those based upon the electrionic principle. On the other hand, the microphone made of carbon or other analogous material gives rise to sufficiently powerful efi'ects in order to be able to realize an amplification sufficiently great practically for all needs and requirements by the arrangement, cascadefashion, of a relatively small number of amplifiers of convenient type. However, such a microphone alters more o r less markedly the sounds that are transmitted so that the receiving apparatus fails to reproduce in a faithful manner the accoustic vibrations produced at the sending station. Now, by profiting from the existence of modern amplifier devices it has been suggested to replace the (or analogous telephone) operating like a generator (which is the original system evolved by Bell). But in this case the weakness of such a generator makes it imperative to have recourse to such a great number of amplifiers disposed cascade-fashion that:

U. S. Patent No. 1,687,253, Oct. 9, 1928) we.

know already a means of forming such an impedance, namely, by means of two thermionic tubes united in a convenient manner. However, its practical usefulness has heretofore been demonstrated only in the case where a negative purely ohmic resistance shall be constructed (diminution of decrement or damping of circuits).

If, in accordance with the present invention, one examines the case of a telephone of the generator type Working upon an inductive circuit, the use of a negative resistance purely ohmic in nature will soon be limited, for we would thus have to deal with a circuit whose apparent time-constant would become microphone by an electromagnetic telephoneexcessively high. On the other hand, by taking also a negative inductive reactance, the said time constant will be kept inside permissible limits so that the compensation of the ohmic resistance of the circuit itself can be carried very much farther, and as a consequence the amplification. Be it well understood, for the purpose of simultaneous compensation of the resistance and the -reactance of this circuit, there could be adopted either the same proportion or else a different proportion, this being always feasible with the Latour apparatus before referred to.

Moreover, the scheme just described could, if need be, be utilized with any kind of telephonic sender or transmitter, for instance, the electrostatic microphone, etc., or even with the ordinary kind of microphone. Generally speaking, the above scheme allows of realizing a really and truly aperiodic amplifier device, that is to say, amplifying in the same manner currents of any given frequency at all, possibly also'ofl the ultraac.

coustic kind.

In the French patent application filed on October 28, 1922, under the provisional Number 165,525, and entitled Improvements in telephone receiving apparatus, and in the addition thereto dated December 5, 1922,: the inventor has indicated the advantages of the combination with an electromagnetic telephone receiver device of negative artificial impedances, or even of filters and resonators, for the object particularly of realizing a faithful reproduction of the sounds transmitted.

Now, it may be pointed out that such combinations are likewise applicable, and quite generall so, in the case of a sender telephone. Especi 1y is it possible in this manner to obtain at the terminals of. oneof the elements of the Acomplex circuit wherein the said telephone is inserted a potential whose amplitude or crest is strictly proportional to the amplitude of the force coming to act upon the diaphragm, regardless of what may be the frequency of vibratory movement under consideration.- This tension could be applied to the grid circuit of a convenient amplifier amplifying the effect without alteration preferably of the sound transmitted.

As to the rest, one often uses as such send ing telephone a device whose diaphragm (or vibratory organ) possesses a natural frequency of vibrations .that is higher than the usual frequencies met with in connection with the transmission of speech or of music. -In

. sistance and the reactance of the circuit con- "vibratory or taining the sending telephone, and the tension utilized would be obtained at the terminals of acondenser inserted in the said circuit, etc. Moreover, if the said condenser should offer a reactance sufficiently high to the highest .frequencies utilized in telephony workythfe'icompensation referred to will not Abe indispensable. And furthermore, if the l an possesses a very high natural frequency o vibrations, the negative resist- :ance would not be obligatory, under this condition, however, that the circuit possesses a tota-l inductance that is comparatively high as compared with the total resistance, the latter comprising an auxiliary resistance insuring the coupling with the grid circuit possessing `itselfa certain conductance, coupling can be accomplished under all circumstances by means of a transformer involving a magnetizing current as small as feasible.

e invention is illustrated diagrammatically in the accompanying drawing in which Fig. 1 shows a circuit arrangement conformlng to the invention, and

Fig. 2 shows a modification thereof. Referring to Fig. 1 an electro-magnetic transmitter is represented by T comprising a vibratory magnetic diaphragm 1 and a permanent magnet 2 having coils 3 wound thereon. Sound waves impinging on the magnetic diaphragm will cause it to vibrate, thereby V v bungee 'o `Ina'ignetic flux therein,

auf U Y :.c

' the coils 3 cori to' thesoun wavesf'mpinging on the diaphragm. l The coils 3 areconnected in series so that the voltagesinduced therein W11l add and one free terminal is connected to one terminal of the negative impedance represented generally by F, the other terminal of which is connected to the primary of coupling 14. The free ends of the primary of coupling 14 and the coils 3 are connected to form a closed circuit-including coils 3, device F and primary of coupling 14. The secondary of transformer 14 1s connected across the input circuit of a vacuum tube amplifier 16 in whose output circuit is connected a .sound responsive device 17.

1t would be necessary to compensate the re-` .1t can, of course,

apofthe magnet '2mdk-across the input circuit of the first e negative impedance F constitutes an V arrangement described in the French and U. S. patents to Latour 501,472 and 1,687,253, respectively, referred to abovef This device comprises two vacuum tubes 6 and 10 resistance coupled in cascade. circuit of the tube 6 is connected an impedance represented in this case by an inductance 4 and condenser. The output circuit of this tube contains plate batteries 7 and 9 and a resistance 8. The potential of the battery 9 is such as to normally compensate the voltage drop of the plate current in resistance 8 so that the potential difference bet-Ween the points A and B is normally zero. In like manner the potentialdifference be- Across the inputtween the points C and D in the output circult of tube 10 is'normally zero. Now, if

a current traverses the impedance 4---5 a cuit of the grid filament circuit of tube 6 which will be amplified by tubes 6 and 10 and appear as a negative voltage drop between the terminals C and D. It is obvious therefore, that the device F acts as a negative impedance and that this negative impedance can be regulated by a suitable choice of the impedance introduced in the input'circuit of the first tube 6. It will be clear also that the impedance comprising the inductance 4 and condenser 5 can be so chosen as to give in the primaryof the transformer 14 coil changes corresponding as closely as desired to the sound wave originally impressed on the diaphragm 1. In other words the'device F acts l as a combined filter and amplifier. Having proportional t the impressed' sound waves be amplified to any desired extent whereby an intense and faithful re-- production of the original sounds will be obtainedin the reproducing device 17.

In Fig. 2-is shown a slight modification of the device shown in Fig. 1. In this case the device F is replaced by the device F and the transformer 14 is dispensed with. Here the transmitter coils 23 are connected -directly tube 26 and th 35 -is connected across the resistance 32 and battery 33 in the output circuit of the tube 30. tially the same as that shown u in Fig. 1, that 1s, normally battery 29 will serve to compensate for the voltage drop caused by the plate current in resistance 28 and'there will' .beo voltage impressed on the input circuit of tube 30 in the absence of vibrations of the diaphragm 31. In like manner the battery 32 compensates for the voltage drop in resistance v33. -As before, the device F serves as a iilter and amplifier in the coils 23 to give a faithful reproduction of the original sounds impressed on the diaphragm 21.

It will be obvious that in both these connece input-circuit of the amplifying tube The operation of this device is substanforv the voltages induced.`

tions the impedance in the in ut circuit of the first ltube 26 can be suitably chosen to obtain from the output circuit of the second tube of this filtering and amplifying device voltages Which Will correspond with the sound Waves impressed on the diaphragm 21 regardless of any natural periods of vibral tion of this diaphragm, whereby the faithful and intense sound reproduction may be accomplished.

Having described my invention, what I claim is:

l. In an arrangement for reproducing sound vibrations, a transmitter having a coil responsive to the vibrations, and means for compensating the fall of potential across said coil, said means including a vacuum tube resistance-coupled amplifier having its grid connected to one end of the coil and the output circuit connected to the other end of the coil through the primary of a transormer, and a series-resonant filter circuit connected across the input circuit of the resistance-coupled amplifier for selectively controlling the amplitude of certain frequencies impressed on the amplifier.

2. In a device of the class described, the Y qcombination With an electromagnetic transmitter provided with a coil, of a negative impedance device for compensating the fall of potential across said coil, said negative impedance device including a pair of resistance- J@coupled amplifiers, and a resonant circuit shunted across the input of the first amplifier. 3. In a device of the class described, the combination with an electromagnetic trans mitter provided with a coil, of a negative impedance device for compensating the fall of potential across said coil, said negative impedance device including an amplifier comprising one or more electron discharge tubes, and a series resonant circuit shunted across the input of said amplifier.

4. In an arrangement for reproducing sound vibrations, a transmitter having a coil responsive to the vibrations, a negative impedance device Jfor compensating the fall of potential across said coil, said negative impedance device including an amplifier,'and a series resonant filter 'circuit connected across the input circuit of said amplifier for selectively controlling the amplitude of certain frequencies impressed on the amplifier.

JOSEPH BETHENOD. 

